Electric motors are used in numerous applications, including, but not limited to applications in which the desired action is to position components in response to a command signal. The devices for these applications are classified as electrical actuators that may incorporate some form of mechanical advantage, such as gear reduction, to multiply the motor torque. In addition, a return spring is often used to position the actuator to a default position when no current is applied. These actuators typically use a DC motor with permanent magnets, either on the rotating or stationary member, to provide a magnetic field which the current carrying windings of the motor can react to produce useful torque and motion.
Motors have several characteristics that effect their operation. A first characteristic is the amount of conductor material (usually copper) that can physically be used for a given motor size. Typically a motor will have several regions of conductor material wound about a stator. In general increased winding mass decreases the electrical heating loss and the motor can deliver greater torque. The second factor in defining the motor operation is non-energized or detent torque. This is the amount of torque required to rotate the motor without current. This torque is caused by the interaction between the magnetic force of the individual magnetic poles of the rotor with the magnetic poles of the stator as a function of angular position. A low detent torque will minimize the electromagnetic motor torque required to rotate the motor. In practical applications this could allow a return spring torque to be reduced which would increase useful torque and minimize motor heating.
FIG. 1 is a cross-sectional plan view of a motor 10 as known in the prior art. The motor 10 has a stator 12 that contains the portion of the motor 10 that is actuated. Within the stator 12 is a movable rotor 14 that rotates within the stator 12. The rotor 14 has poles 16 spaced along its outside circumference. The stator 12 functions to interact with the poles 16 on the rotor 14. As shown in FIG. 1 there are three coil center poles 20 with alternating outside winding poles 22. An air gap 26 is positioned between each coil center pole 20 and the poles 16 of the rotor 14 and each outside winding pole 22 and the poles 16 of the rotor 14. The coil center poles 20 are configured to receive a bobbin 21 having an electromagnetic coil 23 wound upon the bobbin 21. The bobbin 21 is configured to slide over and surround each center coil pole 20. Each outside winding pole has a tooth 24 that functions to balance the magnetic forces generated by the coil center poles 20. The problem with this design is that the tooth 24 inhibits the width of the bobbin 21 and electromagnetic coil 23 that can be placed about the coil center poles 20. Therefore it is desirable to have a motor design that will allow for a wider bobbin 21 and coil 23 to be used.